The replication of the Escherichia coli chromosome is being investigated. Autoradiography and DNA-DNA hybridization are being used to determine if replication is always bidirectional, or if replication is unidirectional under some growth conditions. The replication of the E. coli chromosome by the integrated prophage lambda (xis) is being used to determine the state of the DNA at the replication origin and terminus. The genetic location of the replication origin and terminus is also being determined.